Corrosion of processed metals, such as steel, copper, and zinc, is a process whereby elemental metals, in the presence of water and oxygen, are converted to oxides. Although corrosion is a complicated process, it may be considered an electrochemical reaction involving three steps which occur at the anodic and cathodic sites of a metal surface, as follows:
1. Loss of metal to the water solution in oxidized cationic form at an anodic site, with concomitant release of electrons ("anodic reaction");
2. The flow of the released electrons to a cathodic site; and
3. Oxygen at a cathodic site uses the electrons to form hydroxyl ions ("cathodic" reaction), which flow to an anodic site.
These three basic steps are necessary for corrosion to proceed, and the slowest of the three steps determines the rate of the overall corrosion process. The cathodic reaction is often the slowest of the three steps because the diffusion rate of oxygen through water is slow.
A corrosion control program usually depends on specific inhibitors to minimize the anodic or cathodic reaction, or both. Among the various types of corrosion inhibitors are organic compounds, which act by adsorbing or chemisorbing as thin layers on metal surfaces to separate the water and metal. These materials form and maintain a dynamic barrier between the water and metal phases to prevent corrosion. One series of compounds applied to reduce copper and copper-alloy corrosion are aromatic organic corrosion inhibitors. This series of organic compounds, which includes mercaptobenzothiazole ("MBT"), benzotriazole ("BT"), butylbenzotriazole ("BBT"), tolytriazole ("TT"), naphthotriazole ("NTA") and related compounds, react with the metal surface and form protective films on copper and copper alloys. These compounds are active corrosion inhibition treatment components and are referred to generally herein as copper corrosion inhibitors or corrosion inhibitors, or as aromatic azoles, and at times as triazoles or aromatic (thio)(tri) azoles.
Active components may be lost due to deposit, corrosion, chemical and microbiological degradation processes and physical losses (blowdown, drift, incorrect feed rates, and the like) and combinations of such phenomena, which are discussed in more detail below. Monitoring the loss of an active treatment component, particularly if such monitoring permits the extent of loss to be quantified and if the monitoring is continuous, is an indicator of treatment program performance. Moreover, if such monitoring is continuous and the active component loss clearly determined, automatic control permits the corrosion inhibitor losses to be compensated for an dosage to be precisely controlled.
The conventional analytical procedure for analysis of copper corrosion inhibitors is a UV(ultraviolet light)-photolysis/photometric method, and is well known to persons of ordinary skill in the art. This method, however, has a number of limitations. It is not well suited for continuous monitoring and/or control. It provides results that are strongly dependent upon the operator's laboratory technique. It cannot distinguish the chemical structure of the aromatic azole that is present. Its observed response is non-linear with respect to aromatic azole dosage. It requires that the aromatic azole be degraded with an ultraviolet lamp in the presence of a color-forming reagent. The UV-photolysis/photometric method and its limitations are discussed in more detail below.
It is an object of the present invention to provide a method for monitoring and/or controlling copper corrosion inhibitor losses and/or dosages on a continuous basis. It is an object of the present invention to provide a method for monitoring and/or controlling copper corrosion inhibitor concentrations that is substantially independent of an operator's laboratory technique. It is an object of the present invention to provide a method for monitoring and/or controlling copper corrosion inhibitor concentrations that can distinguish the chemical structure of the aromatic azole that is present. It is an object of the present invention to provide a method for monitoring and/or controlling copper corrosion inhibitor concentrations that provides a response that is substantially linear to aromatic azole concentration. It is an object of the present invention to provide a method for monitoring and/or controlling copper corrosion inhibitor concentrations that does not require UV digestion of the aromatic azole. These and other objects of the present invention are described in more detail below.